Compositions capable of being converted under the influence of actinic light to solid, insoluble, tough structures have become increasingly important in the preparation of printing plates. U.S. Pat. No. 2,760,863 to Plambeck is related to such compositions. In the process of the Plambeck patent, printing plates are produced directly by exposing to actinic light through an image bearing transparency a layer of an essentially transparent polymerizable composition containing an addition polymerization initiator which composition cross-links on exposure to actinic light. The layer of polymerizable composition, which is supported on a suitable support, is exposed until substantial polymerization of the composition occurs in the exposed areas while substantially no polymerization occurs in the unexposed areas. The unchanged material in the latter areas is then removed, as by treatment with a suitable solvent in which the polymerized composition in the exposed areas is insoluble. The result is a raised relief image, corresponding to the transparent areas in the transparency, which is suitable for use as a printing plate, as in letterpress and dry off-set work
Photopolymerizable layers of the type described by Plambeck generally are prepared from polymeric components which are soluble in organic solvents. Due to the toxicity, high volatility and generally low flash point of low cost organic solvents, their use often gave rise to hazardous conditions. As a consequence, further research was directed toward the development of photopolymer compositions which are soluble in water or aqueous alkali. In U.S. Pat. Nos. 3,794,494 to Kai et al and 3,960,572 to Ibata et al, the disclosures of which are both hereby incorporated by reference, water or aqueous alkali dispersible compositions are described which are suitable for use in rigid or elastic flexographic printing plate preparation. These compositions comprise polyester-polyether urethane-linked block polymers containing acrylate or methacrylate end groups (U.S. Pat. No. 3,960,572), or unsaturated polyesters (U.S. Pat. No. 3,794,494) in combination with unsaturated monomers and a photopolymerization initiator. However, the individual compositions of U.S. Pat. No. 3,960,572 and U.S. Pat. No. 3,794,494 generally are not suitable for making printing plates to be used with the type of inks containing acrylate monomers with no solvents which are cured by ultraviolet radiation. Such ultraviolet inks require soft printing plates to provide maximum ink transfer and print quality. Soft plates made from unsaturated polyester compositions generally have poor physical properties and do not withstand the printing or plate cleaning processes, while the polyester-polyether urethane-linked block polymer formulations produce plates which swell and disintegrate when used with such ultraviolet inks and plate cleaning materials.